This invention relates to apparatus useful in the field of large boilers of the type that are used by electric utilities to generate electricity and in other industrial applications. In such uses, coal has been a traditional and desirable fuel source economically and strategically because of its great abundance, comparative low cost, and widespread availability in the United States. More recently, natural gas and oil distillates have become used as fuel sources, particularly as greater emphasis has been placed on reducing contaminants and other unwanted constituents from the effluent gases that are produced in quantity by such installations when they use coal. That trend and efforts to improve the efficiency of power plants have led to the concept of using combustion gases directly to drive generating turbines without going through the intermediate step of using the fuel to generate steam first. Thus conventional generating systems use the Rankine Cycle, wherein coal is used to generate steam which then drives the turbine generators, have been combined with so-called direct fired Brayton Cycle systems, where a fossil fuel heat source drives a turbine directly without the intermediate step of steam generation, with the remaining heat in the spent gas from the direct fired Brayton cycle being used as an energy source for the associated Rankine cycle system. Systems having such combinations of direct fired Brayton and "bottoming" Rankine systems are generally referred to as Combined Cycles. While more efficient thermodynamically, a major drawback of this approach is that currently gas turbines or direct fired Brayton cycle systems are generally not adapted to the use of solid fuels because of the high probability of the deposition of ash on the blades of the turbine which occurs if solid fuels are used at normal operating temperatures to heat directly the gas which drives the turbine. To avoid this, comparatively expensive and strategically more critical fuels, such as natural gas and distillate fuels, have had to be used, since alternative approaches to the traditional methods of burning coal so as to use it as the direct heat source in such Brayton Cycle systems have also proved to be unsatisfactory. For example, coal gasification with removal of the ash constituents is cost competitive only for large size plants. Alternatively, subjecting the coal to pressurized, fluidized bed processes produces maximum temperatures about 1750 F which is far below the 2300 F needed to satisfy the needs of a modern gas turbine to achieve high efficiency in the operation of the gas turbine. More recently, there has emerged an alternative approach to heating the turbine working fluid in which the fluid would be heated indirectly or through a heat exchanger. Power plants of this type have been studied since the 1930's in an effort to utilize high thermally efficient gas turbine cycles with solid ash bearing fuels. This approach, referred to as an Externally Fired Combined Cycle ("EFCC"), utilizes a heat exchanger as a means to transfer heat to the gas which impels the turbine while, at the same time, isolating the ash and other contaminants from the turbine itself. In this concept, taken, for example in the context of turbine generator power plant, clean, filtered air is admitted into the compressor section of an externally fired gas turbine where it is pressurized and raised to a temperature of about 375 degrees (C). This flow in the preferred embodiment becomes the tube-side flow through a shell and tube heat exchanger, where the air in the tubes is raised by transfer of energy through the tubes to a temperature of about 1200 degrees C. (approximately) and then admitted into the turbine section where it is expanded to drive the turbine and generate electricity. This gas exits the turbine at about 540 degrees C. and at a slight pressure above atmospheric, with part of it being supplied to a solid fuel (e.g., coal) combustor, where the energy supplied by the fuel raises the gas temperature to above 1350 degrees C. The products of this combustion process flow through the shell side of the heat exchanger and there become the source of heat that is imparted to the high pressure compressor discharge air in the tubes. From the shell side of the heat exchanger, the gas flows into the heat recovery steam generator comprised of one or more superheaters, evaporators and economizers. As noted above, a chief difference between the indirect approach and the earlier direct concept is the elimination of the introduction of combusted fuel gases containing ash into the turbine. That is, the ash and other contaminants are kept from the turbine blades and other elements of the interior of the turbine comprising the gas path, since the air from the EFCC which the turbine "sees" is isolated from the combustion of the external firing by the interposed heat exchanger.
In the earlier, indirect fired systems, operating temperatures were much lower than those to which the technology has now evolved, so that high temperature alloy steel air heaters could perform reliably. However, metallic heat exchangers do not permit sufficiently high temperatures to satisfy the requirements of today's high performance gas turbines, particularly the so-called aircraft derivative machines that have been developed for industrial use. The use of ceramic air heaters can circumvent this obstacle since ceramics can endure temperatures well above 1370 C. in the chemically harsh environment produced by the combustion of coal. The physical properties inherent to ceramic materials make tube type heat exchangers the preferred form of such structures for such uses, and experience has shown them to exhibit good durability. However, when so applied, the ash build-up which occurs on the tubes progressively inhibits their efficiency as heat exchange elements. This indicates a need for an ash collection system "up-stream" of the heat exchanger in EFCC installations to avoid such ash build up on the ceramic tubes.
Accordingly, it is an object of this invention to provide structures to collect ash from a stream of gas. It is a further object of this invention to provide such structures which are particularly adapted for use in collecting coal ash. Yet another object of this invention is to provide means which will satisfy one or more of the foregoing objectives that is adapted for use in high temperature environments. Still another object of this invention is to provide means which will satisfy one or more of the foregoing objectives and is self-cleaning.